1. Field of the Invention
The present invention relates to a driver for driving a light emitting element, the driver being capable of performing testing of the light emitting element. In particular, the present invention relates to detecting whether any channel of the driver is in an open or short condition.
2. Discussion of the Related Art
Recently, LEDs have become a popular source of light in a broad variety of applications. For instance, power LEDs have been employed as general lighting as well as for road work signs, which may be battery operated or solar powered, and also for traffic displays. LEDs may be further found in electronic goods as well as in gaming machines. In addition, LEDs represent a very efficient means for display backlighting. Full color or monochrome LED matrixes are further used for high resolution giant video displays.
In order to drive LEDs, LED drivers are used, which typically provide a plurality of output channels for driving a plurality of LEDs. An LED driver for a particular number of channels may be implemented, for instance, as an integrated circuit embedded on a chip. A plurality of such drivers may be employed in a cascade in order to enable the driving of a higher number of LEDs.
An advantageous feature of an LED driver lies in its capability of detecting short and/or open output errors. Typically, various conditions are tested on the output line such as open line, short to ground (GND) or short to Vo. Recently, LED drivers with such detection functionality have been developed and introduced on the market. For instance, STMicroelectronic product sheet STP16DPP05 (available at www.st.com) relates to a low voltage 16-bit constant current LED sink driver with output error detection. The driver of this document does not require increasing the pin count for the purpose of output error detection. Rather the existing pins are assigned a secondary function. A dedicated logic sequence on predefined pins allows the device to enter or exit from the detection mode. For instance, pins such as an output enable pin (OE) and the latch enable pin (LE) may be input a logic sequence of a predetermined duration of clock (CLK) cycles in order to switch the controller from the “normal mode” to the “error detection” mode.
In the error detection mode, an internal measurement of voltage and/or current from all the channels is performed. Thus, in order to detect a faulty condition, all channels should be ON. In a conventional LED driver, the channels are set to the ON state by setting all the outputs to logical “one”, which may be performed, for instance, by means of a serial input pin (SDI). The LED driver drives the LEDs after the output enable (OE\) signal is set to an active low level, in order to analyze whether an open or short condition has occurred. During the time in which the output enable signal is low, it is possible to perform the measurement of voltage and/or current in order to detect an error as described, in particular, in Section 7 of the STP16DPP05product sheet.
Typically, the status of the LEDs is detected during a predefined error detection time. After this time period has elapsed, the circuit controlling the LED driver, for instance a microcontroller, resets the output enable signal (OE/DM2) to a high state. Then the output data detection result is sent to a serial output line (SDO). Typically, error detection mode and normal mode both use the same data format. As soon as all the detection data bits are available on the serial output line, the device may return to the normal mode of operation.
Re-entering the normal mode may be performed in a similar way to entering the detection mode, namely by inputting one or a plurality predefined pins such as OE/DL2 and LE/DM1 a predefined logical sequence within a predefined number of clock pulses.
FIG. 5 is a block diagram which illustrates a simplified functional structure of a driver for at least one LED according to the state of the art. An LED 850 is driven by a channel driver 820 by means of its output signal Iout 830. The channel driver 820 is configured to drive the LED 850 according to an OE\ signal 810. The channel driver and the testing means may also be controlled via other input signals than the OE\ signal, as described above. Moreover, the Iout signal 830 is tested by a tester 840 in order to determine whether the LED 850 is in a short circuit, or in an open circuit condition. For instance, by measuring the Iout signal corresponding to 0, or lower than a predetermined value, it may be concluded that the LED 850, or the connection to the LED 850 is in an open circuit condition. Alternatively, for instance, by measuring a current higher than a predetermined threshold it is possible to conclude that the LED 850, or the connection to the LED 850 is in a short circuit condition. However, in order to perform such a testing process, the Iout signal is preferably in a steady state. The block diagram in FIG. 5 represents the functional structure of the driver. The channel driver 820 and the tester 840 may in fact be integrated in a single chip as discussed above. In particular, the switching between the normal mode and the testing mode of the driver may be performed by a predefined logic sequence input to one or a plurality of pins of such a driver.
The duration of the error detection period necessary for performing the measurement, corresponding to the low state of the OE\ signal typically depends on parametric conditions such as voltage, temperature and process spread.
FIG. 4 illustrates a typical timing for performing the measurements. Upon switching the Output Enable (OE\) signal 706 at a time instant t0 to an active low state, the output current Iout 705 of each channel of the driver rises until it reaches a steady state value. This rise is not instantaneous but rather requires a time period Trise 702. The measurement of current in order to detect an open or short condition is only reliable after reaching the steady state. The time period Trise 702 depends on many factors such as the working voltage, temperature and process spread.
Moreover the internal circuitry of the driver performing the measurements requires a time period Tmeas 703 for performing the measurement. In order to perform the error detection reliably, the OE\ signal should thus be kept low for at least a time period Terr 701 given byTerr=Trise+Tmeas.
If the OE\ signal 706 remains in the low state for a time period shorter than Terr 701, the result of the detection may be incorrect. Thus, in order to reliably detect an open or a short condition, the ON time of the OE\ signal, corresponding to a low active state, has to be greater than Terr 701.
In order to determine the ON time of the OE\ signal required for correctly performing the measurement, it is thus necessary to take into account the time period of signal rising Trise 702 and the time period necessary for performing the measurement Tmeas 703. However, both these time periods are significantly dependent on parametric conditions. Therefore, vendors of LED drivers with capability of detecting a short and/or open error condition usually provide a worst case condition in the specification of the driver, which is the time period Terr—MAX 704 necessary for error detection in the worst case. The user then has to wait until the time period corresponding to Terr—MAX 704 has elapsed in order to consider the test process completed and in order to read out the detection results.
Consequently, the time for the error detection is usually oversized, resulting in the LEDs being turned on for a longer time than effectively needed. However, for the majority of industrial applications, it is desirable to keep the error detection time as low as possible, in particular in cases such as LEDs with deep dimming.